This invention generally relates to a method and apparatus for detecting the endpoint in chemical-mechanical polishing of semiconductor wafers; and more particularly, the invention relates to detecting the endpoints of critical areas on the wafer.
In the production of ultra-high density integrated circuits, chemical-mechanical polishing (CMP) processes are used to remove material from the surfaces of wafers. In a typical CMP process, a wafer is pressed against a polishing pad in the presence of a slurry under controlled chemical pressure, velocity, and temperature conditions. The slurry solution generally contains small abrasive particles that abrade the surface of the wafer, and chemicals that etch and/or oxidize the surface of the wafer. The polishing pad is generally a planar pad made from a continuous phase matrix material, such as polyurethane. Thus, when the pad and/or the wafer moves with respect to the other, material is removed from the surface of the wafer by the abrasive particles (mechanical removal) and by the chemical (chemical removal) in the slurry.
When a conductive layer is polished from a wafer, the CMP processes must accurately stop polishing the wafer at a desired endpoint. Conductive layers are typically deposited over insulative layers to fill vias or trenches in the insulative layer and to form electrical interconnections between device features on the wafer. To isolate electrically the interconnects from one another, it is desirable to stop the CMP process below the top of the insulative layer and above the bottom the conductive material in the vias and trenches. If the CMP process is stopped before the desired endpoint ("under-polishing"), then the interconnects will not be electrically isolated from one another and shorting will occur in the circuit. Conversely, if the CMP process is stopped after the desired endpoint ("over-polishing"), then interconnects may be completely removed from the wafer. Therefore, to avoid serious defects in a wafer, it is highly desirable to stop the CMP process at the desired endpoint.
It is particularly difficult to determine the endpoint of the CMP process on wafers that have small "critical areas." The critical areas are typically depressions on the surface of the wafer that are the last point on the wafer from which the conductive material is removed by CMP processing. The location and size of the critical area is a function of the circuit design and the uniformity of the polishing rate across the surface of the wafer. As a result, the critical areas vary from one type of die to another, and they typically occupy a minuscule portion of the wafer surface.
Metal CMP uniformity and performance (i.e. meeting the process parameter specs) can be controlled by carrying out R.sub.s (sheet resistance) measurements and measuring the yield of open/short yield macros after CMP processing. However, such techniques generally require removal of the wafer from the CMP processing area with the need to reinstall the wafer in the CMP unit in the event polishing is measured to be incomplete. Optical thickness measurements as commonly used in oxide CMP are not possible because of the reflectivity of metal films. Profiler measurements are time consuming and are highly affected by the topography under the feature to be profiled. Thus, there is a need for improved methods of judging the uniformity of polished wafers while the wafers are still in the CMP area.